CLEVELAND -- The Indians' front office spent a lot of time dissecting the production turned in by Yonder Alonso last season. If he was going to be Cleveland's new first baseman, the team's evaluators needed to believe that what Alonso did last year was real and not a one-season anomaly.
It is well documented that Alonso underwent changes to his swing mechanics and adopted a more aggressive mentality last year. What Cleveland needed to do -- before signing him to a two-year, $16 million contract Dec. 23 -- was delve into the data to determine whether those adjustments were sustainable. The Indians came away convinced the results were not a fluke.
"It was a purposeful adjustment that Yonder made," said Chris Antonetti, the Indians' president of baseball operations. "It's also not a one-year trend. If you look at the underlying numbers for him, he's been a guy who has typically controlled the strike zone, been patient at the plate, and this year he made an adjustment to that approach and hit for some more power. That's also continuing a trend of building on improved exit velocities over the course of the last three or four seasons."

Part of Alonso's altered approach last year was aimed at hitting the ball harder and getting pitches into the air more often, making him one of the poster boys for the so-called fly-ball revolution. That kind of strategy can come with some sacrifices, though. As Antonetti mentioned, Alonso's average exit velocity has indeed increased over the course of the past few years, but his contact and strikeout rates have suffered in the process.
Here's a look at Alonso's change in exit velocity, per Statcast™:
2015: 87.7 mph
2016: 88.2 mph
2017: 89.2 mph
And here is how his average launch angle has evolved:
2015: 7.6 degrees
2016: 10.3 degrees
2017: 19.4 degrees
Along the way, Alonso's swinging-strike rate increased as follows:
2015: 6.6 percent
2016: 7.4 percent
2017: 11.4 percent
Now, as long as the contact made is having a larger offensive impact, a player and his team can live with more whiffs (his 22.6 percent strikeout rate was his highest in a full season). What Alonso showed last year was that his increased fly-ball rate (43.2 percent in '17 compared to 33.3 percent in '16) was leading to more run production, even with his swinging-strike rate climbing and contact percentage down (75.6 in '17 vs. 83.9 in '16).
Overall, Alonso turned in a .266/.365/.501 slash line with a career-high 28 home runs and 2.4 WAR (per FanGraphs). He did a lot of his damage against fastballs, posting a .613 slugging percentage against heaters in '17, compared to a .411 slugging against fastballs in '16. Alonso had a 1.000 OPS on June 22 with a 50.7 percent fly-ball rate through that point of the season in 234 plate appearances.

From there, Alonso encountered some turbulence both on and off the field. Pitchers began decreasing fastball usage against Alonso and tried to exploit different areas of the strike zone against him. There was also the human element of an uncertain future. Trade rumors swirled and came to fruition Aug. 6, when the A's shipped him to the Mariners. On top of that, Alonso was not sure where free agency would take him this offseason.
After June 22, Alonso hit .242 with a .756 OPS and 37.2 percent fly-ball rate in his final 287 plate appearances.

"I think it was a mixture of a lot of things," Alonso said. "Outside the game, a lot of things were going on. I have a family and I didn't know where I was going to be tomorrow and it was tough on that part. And then getting ready for the game was hard as well. For me, it was just learning what guys were doing to me, adjusting to that and then once I got adjusted to that, I was able to be healthy and my mind was healthy."

Another metric that might suggest that Alonso's showing last year was no fluke was the difference in his weighted on-base average (.376) and expected wOBA (.368). When xwOBA -- a metric that accounts both for defense-independent outcomes of batted balls based on exit velocity and launch angle as well as strikeouts and walks -- is close to a player's wOBA, that indicates that what took place on the field is similar to what should have happened based on quality of contact.
Alonso also pointed out that he bounced back over the season's final six weeks, following an offensive lull that stretched from late June through early August. From Aug. 12 on, Alonso hit .288 with an .865 OPS, 40.2 percent fly-ball rate and a 138 weighted Runs Created Plus (indicating he was 38 percent above league average in that stretch).
"Any time a player makes an adjustment like Yonder did," Antonetti said, "the league is going to try to adjust back to him. He still was able to find some degrees of success in the second half with Seattle, but pitchers made some adjustments. We're confident that the approach and changes he made will translate into continued success moving forward."